1. Field of the Invention
This invention relates to a method of manufacturing a control grid used in a fluorescent display device.
2. Description of the Prior Art
A fluorescent display tube effects a luminous display of numerals or figures by selectively impinging electrons emitted from a filamentous cathode when energized and heated upon a plurality of anodes on which phosphor layers are deposited.
As shown in FIG. 1, the fluorescent display tube includes anodes 2 having phosphor layers deposited thereon which are arranged on a substrate 1 in the form of letter "8", filamentous cathodes 3, and control grids 4 which are disposed between the anodes 2 and the cathodes 3 for accelerating and controlling electrons emitted from the cathodes and uniformly impinging the electrons upon the anodes. The grid 4 includes a net portion 4a through which the electrons pass and a frame 4b for supporting the net portion 4a. A lead terminal 4c for applying a control voltage to the grid 4 and a strip 4d for holding the grid 4 on the substrate may be provided with the grid 4 as shown in FIG. 1. Reference numeral 5 designates a front cover having a transparent viewing window portion. The front cover is of flat bottom boat shape and hermetically bounded to the substrate 1 at the peripheral portion thereof to form an evacuated envelope within which to contain each of the electrodes in a high vacuum state. The fluorescent display tube further includes outer terminals 6 for applying energizing potentials to each of the electrodes which are airtightly extended to the outside of the envelope through a sealing portion of the substrate 1 and the front cover 5.
As is apparent from FIG. 1, the rate of dimensions of openings to predetermined regions of the net portion 4a, namely, opening rate of the net portion is preferable to be higher, because light emitted from anodes 2 is observed through the net portion 4a of the grid 4. However, as a matter of fact, a wire diameter for forming the net portion 4a and the opening rate are determined taking mounting strength of the grid 4 and acceleration and controlling capacity of the electrons by the grid 4 into consideration, and the grid made of wires of 20-60 .mu.m in diameter and 60-80% of the opening rate is generally used in the conventional fluorescent display tube.
In the prior art method, the grid 4 is prepared by etching a thin metal plate of 30-100 .mu.m thick, such as, for example, a stainless steel plate, a Fe-Ni alloy plate or the like in an openwork pattern as shown in FIG. 2. FIG. 2 illustrates the etched net portion 4a consisting of a honeycomb lattice frame having the diameter of "a", and the length "1" at one side of the frame and hexagonal openings 10b which is manufactured by the conventional etching process. The etched net portion 4a shown in FIG. 2 is of 60-80% opening rate as explained hereinabove, which means 60-80% of materials used in forming the net portion 4a are removed and destroyed by the etching. This is extremely wasteful.
The etching is a chemical treatment which requires complicated processes, for instance, controls of chemical liquid, temperature of each of the etching processes and the like. Furthermore, the metal plate used for the producing the grid 4 is limited to iron or alloys of iron group, such as, for example, a stainless steel. In addition, it is extremely difficult to increase the opening rate of the net portion by making the wires of the net portion 4a fine without decreasing the manufacturing efficiency of the grid.